1. Technical Field
This invention relates generally to protection devices for rechargeable cells, including electrochemical cells and super capacitors, and more particularly to devices that interrupt current when pressure builds within a rechargeable cell.
2. Background Art
Rechargeable batteries have become integral components of everyday life. Portable electronic devices like cellular telephones, two way radios and laptop computers rely upon rechargeable batteries for their portability. The rechargeable battery offers a way to slip the surly bonds of wall mounted power supplies and touch the face of the wireless world.
Battery packs generally include a plastic housing, electronic circuitry and at least one rechargeable cell. The cell within the battery pack is the device that stores and releases electrochemical energy. Many of these cells are sealed within cylindrical, aluminum (or steel) cans. Within these cans exist the cell's electrode assembly: electrode materials, a separator to keep the electrodes apart, and electrolyte. One of the most popular cans in use today is known as the “18-650” can. It is so named because it is 18 mm in diameter and 65 mm long.
Some cell chemistry types, like lithium-ion for example, produce gas when they are improperly charged, shorted or exposed to high temperatures. This gas can be combustible and may compromise the reliability of the cell. As such, protection circuitry is placed within the battery pack and about the cell to ensure that the cell is not over charged. The protection circuitry generally consists of integrated circuits and other components. Like any physical system, there is a small possibility that one of the components in the protection circuit may fail in the field. For this reason, the cells themselves often include back-up, or redundant, safety components to ensure that the cell is not overcharged.
A popular way of providing secondary protection for a cell is by way of a current interrupt device. One of the more popular current interrupt devices in use today is recited in U.S. Pat. No. 5,418,082, entitled Sealed Battery with Current Cut Off Means, issued May 23, 1995, to Taki et al., incorporated herein by reference. Such a device is illustrated in FIG. 1.
FIG. 1 is a cross-sectional view of the current interrupt device. A sealed cell has a safety valve 5 made of a metal plate that may be deformed by an increase of pressure within the cell. The current interrupt device 6 is actuated by the deformation of the safety valve 5. An insulating disk 23 is fixed between the safety valve 5 and the cell electrode 1. This disk 23 has a central aperture 21 through which the projection 9 of the safety valve 5 is inserted. The disk further includes gas apertures 22 through which a gas, if and when it is generated by the cell, is passed.
A thin metal plate 24, which is electrically connected to one ribbon lead 7 of the cell electrode 1 is attached to the electrode side of the disk 23 in such a manner as to close the central aperture 21. The safety valve projection 9 is welded through the central aperture 21 to the thin metal plate 24.
Referring now to FIG. 2, illustrated therein is the current interrupt device of FIG. 1 after being actuated by gas within the cell. The safety valve 5 has been pushed up by gas generated within the cell that passed through aperture 22 and placed pressure upon the safety valve 5. The safety valve 5 then deformed by swelling toward the cap 3 of the cell. In so doing, the weld between the safety valve 5 and the thin metal plate 24 is broken, thereby interrupting current flow.
The problem with this invention is the weld (between the projection 9 and the metal plate 24). To begin, an expensive, precision welder is required to make the tiny weld through the aperture 21 of the disk 23. Next, if this weld is slightly too strong, the current interrupt device will not open quickly enough. If the weld is slightly too weak, there will be nuisance opening of the current interrupt device. If an operator manufacturing the current interrupt device errs ever so slightly during the welding process, reliability of the device will be compromised. In short, if the welding process is not an extremely precise, six-sigma or better manufacturing process, the current interrupt device will not function as designed.
There is thus a need for an improved secondary protection device for rechargeable electrochemical cells.